Vapor feed system for easy vaporizable materials to the arc chamber of calutrons



April 96 w. A. BELL, JR.. ETAL 3, 35

VAPOR FEED SYSTEM FOR EASY VAPQRIZABLE MATERIALS TO THE ARC CHAMBER OF CALUTRONS Filed Sept. 21,1964

A 0.250" LD. TUBE 7 KNUDSEN FLOW TRANSITION HIM A: 0.093s I.D. TUBE/ LAMINA LOW CONDUCTANCE FOOT (LITERS SEC) 1 10 lOO i000 10,000

PRESSURE (MICRONS) M Fig. 1.

CHARGE --ARG BOTTLE PLASMA x RIBBON IHEATER 340 CHARGE cooLms RADIATION SHIELD FRAME Fig. 2.

INVENTORS.

ATTORNEY.

United States Patent VAPOR FEED SYSTEM FOR EASY VAPORIZABLE MATEREALS TO THE ARC CHAMBER OF CALU- TRONS William A. Bell, .lrz, and Robert M. Ennis, 3L, Oak Ridge, Tenn., assignors to the United States of America as represented by the United States Atomic Energy Commissicn Filed Sept. 21, 1964, Ser. No. 398,137 3 Claims. (Ci. 25041.9)

The present invention relates to an improved means for more accurately controlling the feed vapor into the arc chamber of mass spectrometer ion sources.

One of the most common occurring problems of production-type mass spectrometers, or calutrons, is that of controlling the feed rate of charge material vapor to the arc chamber. A particular problem exists with materials that are too easily vaporized. Typical is the control of mercury vapor into the arc chamber. This element changes phase so easily, either to a vapor or back to a liquid, that vapor flow is normally uneven either too much or too little. This problem has generally been minimized in the prior art by extremely fine temperature control in a boiler and the subsequent feed line to the arc chamber. Another means for reducing the problem is through the use of an external feed system. In the first of these, any change in heat generation around the ion source causes some feedback of temperature to the boiler, thus affecting the rate of vapor generation. In the second method, mercury often condenses in the long tube leading from the external supply and again vapor flow ceases to be carefully controlled. If sufficient pressure is used, flow becomes laminar and cannot be accurately controlled. Similar problems also have existed with any calutron feed material that has a high specific heat, such as compounds of tin, selenium, tellurium, molybdenum, etc. In special instances, another prior art method that has been utilized to overcome the problem of uneven vapor flow is that of a restrictive tube leading to the arc chamber. After a relatively short period of operation, such restrictive tubing became plugged and, therefore, the unit was no longer operable.

With a knowledge of the above problem of vapor feed to a calutron, it is a primary object of the present invention to provide a vapor feed system for the arc chamber of a calutron wherein the feed vapor can be accurately controlled. I

It is another object of the present invention to provide a vapor feed system for a calutron wherein the vapor flow can be maintained constant at any desired value, without restrictive condensation of feed material in feed tubes.

These and other objects and advantages of the present invention will become apparent upon a consideration of the following detailed description and the accompanying drawing, wherein:

FIG. 1 is a graph illustrating the relationship betwee the conductance of a capillary tube and the applied pressure, and

FIG. 2 is a schematic drawing of the vapor feed system of the present invention.

The above objects have been accomplished in the present invention for a vapor feed system for a calutron by providing a small length of restrictive tubing between a heated charge bottle and an adjustable leak or needle valve and providing a larger diameter tubing between the needle valve and arc chamber, such that the charge vapor has its lowest conductance without allowing laminar flow between the charge bottle and the needle valve and a high Knudsen flow conductance between the needle valve and are chamber.

Referring now to the drawing, in FIG. 2, an ion Patented Apr. 11, 1967 "ice source unit 1 is mounted on a face plate 2 as used in a 255 calutron. Such a calutron is described in report ORNL-3606, issued May 1964. Within the source 1 is a charge bottle or boiler 3 which is provided with a heater 4. The exit of the boiler communicates with a length of restrictive tubing '5 having a high length per diameter ratio. This, in turn, connects to a leak or needle valve 6. The valve 6 is connected to an arc chamber 7 by a relatively large diameter tube 8. The valve 6 is controlled by any suitable means, such as by a rod 9 which extends out through the face plate 2. Any suitable valve can be used, and as an example the valve disclosed in application Serial No. 262,278, filed March l, 1963, now US. Patent No. 3,222,026, issued December 7, 1965, may be used effectively. A conventional, water cooled heat shield 10 is interposed between the charge bottle 3 and the arc chamber 7 to minimize heat feedback from the chamber to the charge bottle.

The restrictive tubing 5 between the charge bottle and the valve 6 has an inner diameter of 0.0938 inch, for example, to limit mass transport in the pressure dependent, transition-flow conductance region. Thus, the pressure in the boiler 3 and restrictive tubing 5 is maintained within the transition flow range of the pressure vs. conductance curve as shown in FIG. 1. Therefore, for the 0.0938 inch ID. tube, the pressure is of a selective value in the range from to 3000 microns to provide constant conductance in the transition-flow conductance region for this size tube. In this pressure range, the conductance per length remains constant; therefore, vapor flow is constant for any valve setting without the attendant breakaway observed at higher pressures and experience by prior art workers.

The diameter of 0.0938 inch for the restrictive tubing 5 has been found to provide satisfactory control of the vapor when the operating temperature is in the range needed to vaporize the compounds HgCl 86:0 and WCl For lower temperature operation the diameter-to-length ratio must be increased but within the limitation of preventing laminar flow. In all cases, the diameter preferably should be as large as possible without permitting laminar flow.

The tube 8, conected between the valve 6 and the arc chamber 7, has an inner diameter of 0.311 inch, for example, and provides for the desired high Knudsen flow conductance in this tube. Thus, by providing a smaller restrictive tubing, 0.0938 inch I.D., between the charge bottle 3 and valve 6, and a relatively larger tube, 0.311 inch I.D., between the valve 6 and the are chamber 7, it is now possible to maintain an accurate control of vapor flow to the arc chamber.

In the prior calutron systems which used an external charge feed system, the restrictive tubing between the charge bottle and valve and between the'valve and are chamber each had the same internal diameter, for example, 0.250 inch. In such a system,the Knudsen flow conductance of the long tube between the valve and are chamber is very low for mercury vapor, and about 10 microns of mercury vapor pressure is required in the charge bottle to achieve a mass transport of 0.25 g./hr. of mercury into the arc chamber at 1 micron pressure. At a pressure of 10 microns, laminar flow occurs in the 0.25 inch tube connecting the external charge bottle to the valve as seen in FIG. 1, and the conductance of vapor from the bottle to the valve increases rapidly with increasing pressure. Since mercury vapor has high enthalpy, a slight increase in system temperature can suddenly change the mass transport of mercury to the valve by a factor of 10 to 100. In turn, the pressure gradient along the length of the 0.25 inch feed tube from the valve to the arc chamber can change by a factor of 10 to and result in almost instantaneous condensation of tens of grams of mercury in the tube. Tank pressure under these conditions becomes high enough to render the separator inoperable. Thus, it can be seen that controlled feed of mercury vapor to the arc chamher by the conventional external charge system, using the same size of restrictive tubing between the charge bottle and valve and between the valve and are chamber, is impractical.

In a typical operation of the present invention, an appropriate feed material (charge) is placed in the charge bottle 3 and the unit prepared for operation in the normal manner. Subsequently, the heater 4 is energized so as to vaporize the charge. Because of the relatively small size of the restrictive tube 5, temperature control of the charge bottle is not as critical as in the prior systems since, over the range of pressure (75 to 3000 microns) for the transition flow conductance region indicated in FIG. 1 for the 0.0938 inch tube, the system has a constant conductance per unit length and thus vapor flow is constant for a given valve setting. Similarly, the larger tube 8 (0.311 inch ID.) can be maintained at a temperature high enough to ensure a continuance of the vapor phase, by means not shown, so as to prevent condensation and subsequent revaporization with the attendant vapor fluctuations in the arc chamber 7. Also, constant and high Knudsen flow conductance is provided by the relatively larger tube 8. Thus, steady operating pressures can be maintained and excellent mass resolution is achieved in the operation of the present invention.

As a result of the operation of the present invention, more uniform and higher outputs (up at least and higher utilization of the charge material occur. The improved results of the present system as compared to a conventional system are shown in the following Tables I and 'II.

TAB LE I chloride form of an element is the best from the standpoint of ionization within the arc chamber. In some instances, however, the chlorides are more difiicult to satisfactorily vaporize or are chemically unstable.

However, the present invention relaxes the close temperature control so as to permit using some of the more difiiculty vaporized compounds. Also, since a rather high vapor pressure exists above the charge material, chemical decomposition is reduced. This overpressure also assists where internal conversion from one compound to another, e.g., an oxide to a chloride, is desirable. Furthermore, the present invention uniquely permits the use of compounds of the precious metals as charge materials where heretofore the only satisfactory results were obtained by electron bombardment of the metal. These various results are illustrated in the foregoing Table 1.

The advantage of accurate vapor control, as obtained with the present invention, can also be exploited when using the calutron for the preparation of cyclotron targets and the like. In this use, an ion beam is utilized to deposit a given quantity of material on a backing plate. In order to prepare a good target, the ion beam must be exceptionally steady. With the present invention for accurately controlling the vapor flow, an ion beam for target preparation has been very closely controlled over the period of bombardment. Similar close control has been accomplished during the preparation of a transistor junction by ion bombardment.

This invention has been described by way of illustration rather than by limitation and it should be evident that this invention is equally applicable in fields other than those described.

What is claimed is:

1. In a calutron provided with an arc chamber, an improved vapor feed control system for feeding vapor to [Comparison of performance of ion source using new feed system with best performance of source using conventional feed system] Maximum Total Ion Output a) Element Feed System Feed Material Average Total Ion Output (ma) Process Efficiency (Percent) Mercury Conventional. H New s- Conventional.

ew Conventional- CS2 New 1 Electron bombardment ion source.

TAB LE II [Comparison of isotopic purities achieved using new ion source charge feed system with best purities achieved at ORNL using conventional feed system] In addition to the above advantages, that is, improved output and isotopic purity achieved with the present system, there are several other advantages of the present invention. Prior research has shown that, in general, the

said are chamber comprising a heated charge bottle for containing an easy vaporizable charge material, an adjustable leak valve, a length of restrictive tubing having a length-to-diameter ratio such as to provide therewithin a constant transition-flow conductance region connected between the exit of said charge bottle and said leak valve, a relatively large diameter tube connected and extending between said leak valve and said are chamber, said larger tube providing high Knudsen flow conductance therein between said leak valve and are chamber, a watercooled heat shield interposed between said heated charge bottle and said are chamber to minimize heat feedback, and means for adjusting said leak valve to maintain a selected pressure in the range from to 3000 microns in said charge bottle and restrictive tubing to provide transition flow in said restrictive tubing for any selected value of pressure within said pressure range, thereby maintain- 5 6 ing accurate control over vapor flow from said charge References Cited by the Examiner bottle to said arc chamber to provide constant vapor UNITED STATES PATENTS flow at any desired value without restrictive condensa- 2882 408 4/1959 Lofgren tion of feed material in said tubes.

-2. The vapor feed system set forth in claim 1, wherein 5 7 References Cited by the Applicant said restrictive tubing has an inner diameter of 0.0938 UNITED STATES PATENTS inch, and said larger tube has an inner diameter of 0.311 2,373,151 4/ 1945 Taylor. inch. 2,393,650 1/ 1946 Metcalf.

3. The vapor feed system set forth in claim 2, wherein 10 said charge material is selected from the group com- RALPH NILSON p'lmary Exammer' prising HgC1 W01 S, 8e0 and RuF W. F. LINDQUIST, Assistant Examiner. 

1. IN A CAULTRON PROVIDED WITH AN ARC CHAMBER, AN IMPROVED VAPOR FEED CONTROL SYSTEM FOR FEEDING VAPOR TO SAID ARC CHAMBER COMPRISING A HEATED CHARGE BOTTLE FOR CONTAINING AN EASY VAPORIZABLE CHARGE MATERIAL, AN ADJUSTABLE LEAK VALVE, A LENGTH OF RESTRICTIVE TUBING HAVING A LENGTH-TO-DIAMETER RATIO SUCH AS TO PROVIDE THEREWITHIN A CONSTANT TRANSITION-FLOW CONDUCTANCE REGION CONNECTED BETWEEN THE EXIT OF SAID CHARGE BOTTLE AND SAID LEAK VALVE, A RELATIVELY LARGE DIAMETER TUBE CONNECTED AND EXTENDING BETWEEN SAID LEAK VALVE AND SAID ARC CHAMBER, SAID LARGER TUBE PROVIDING HIGH KNUDSEN FLOW CONDUCTANCE THEREIN BETWEEN SAID LEAK VALVE AND ARC CHAMBER, A WATERCOOLED HEAT SHIELD INTERPOSED BETWEEN SAID HEATED CHARGE BOTTLE AND SAID ARC CHAMBER TO MINIMIZE HEAT FEEDBACK, AND MEANS FOR ADJUSTING SAID LEAK VALVE TO MAINTAIN A SELECTED PRESSURE IN THE RANGE FROM 75 TO 3000 MICRONS IN SAID CHARGE BOTTLE AND RESTRICTIVE TUBING TO PROVIDE TRANSITION FLOW IN SAID RESTRICTIVE TUBING FOR ANY SELECTED VALUE OF PRESSURE WITHIN SAID PRESSURE RANGE, THEREBY MAINTAINING ACCURATE CONTROL OVER VAPOR FLOW FROM SAID CHARGE BOTTLE TO SAID ARC CHAMBER TO PROVIDE CONSTANT VAPOR FLOW AT ANY DESIRED VALUE WITHOUT RESTRICTIVE CONDENSATION OF FEED MATERIAL IN SAID TUBES. 